JPS597679B2 - Scintillator crystal and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scintillator crystal for converting radiation such as X-rays or γ-rays into light, and in particular to a scintillator crystal made of ZnW04 useful for tomographic imaging devices using the radiation. This invention relates to crystals and methods for producing the same.

In recent years, a narrowly focused X-ray beam is irradiated onto the object from various directions and scanned, the X-rays transmitted through the object in each scanning line are detected, and this is sent to a computer to determine the X-ray absorption rate at each point in the virtual matrix. Virtual Matrix X-ray tomographic imaging devices that display a tomographic image by giving each point a brightness or darkness corresponding to the determined X-ray absorption rate are being actively developed.

Materials containing elements with high atomic numbers (Z) at high density are suitable as scintillator crystals used in the imaging device, and so far, materials such as NaI (Tl), CsI, Bi4Ge3
012, CaWO4 and CdWO, etc. were known.

However, these materials were not satisfactory as scintillator crystals for X-ray tomographic imaging devices.

That is, the scintillator crystal used in the X-ray imaging device is suitable for scanning the X-ray beam in this device,
And is the light receiving device from the scintillator crystal that detects X-rays a conventional photomultiplier? The aim is to switch from a photomultiplier tube (photomultiplier) system to a photodiode system, which not only has high X-ray detection sensitivity, but also has a short afterglow time and a light emission wavelength that is superior to that of a photodiode. It is desirable that the wavelength be in the infrared region of 500 nm or more, which is close to the optimum peak value of .

In contrast, the conventional scintillator crystal described above (
1) In B i 4 Ge 3 02, the X-ray detection sensitivity is 12% of the standard NaI (Tl) as shown in Table 1 below.
; and small.

(2) CaWo4 has a short original wavelength of 430 nm, which is disadvantageous when using a photodiode.

(3) Since C dW04 contains Cd, which is a polluting element,
Countermeasures have disadvantages such as undesirable increases in manufacturing costs.

Note that the X-ray detection sensitivity in the present invention refers to NaI, which has the highest sensitivity among conventional scintillator crystals.
(TI) is 100%.

The present invention aims to eliminate the above-mentioned drawbacks and provide a crystal that is most suitable as a scintillator crystal for an X-ray tomographic imaging device in terms of X-ray detection sensitivity, emission wavelength, and afterglow characteristics.

The present inventors selected ZnWO4 (zinc tungstate) from among the tungstates represented by the general formula MWO, (M: divalent ion), conducted various tests on its properties, and selected crystals for scintillators from among these. We have found this to be particularly useful.

That is, it was known that ZnWo4 has a considerably strong emission intensity in powder form, and that the peak value of the emission wavelength is as long as 520 nm.

However, when we created a single crystal of this substance using a conventional method and investigated its physical properties, we found that the crystal exhibited a dark reddish-brown color and had a large absorption coefficient in the visible light region. As a scintillator crystal, the X-ray detection sensitivity is low at about 5% at most, and it is difficult to use. It didn't become a thing.

Furthermore, the fact that the visible light absorption coefficient is large means that Z nM/
It was thought to be a characteristic unique to 04.

The present inventors have discovered that ZnWO, which is commonly obtained
WO: A crystal that is even more pure than a crystal? When the crystals were grown and their absorption coefficients (at a wavelength of 520 nm) were measured, they found that as the purity increased, the absorption coefficients could be reduced. It was discovered that it can be put to practical use as a crystal.

FIG. 1 shows the absorption coefficient at a wavelength of 520 nm and the X-ray detection sensitivity when a ZnW04 crystal with the absorption coefficient is used as a scintillator crystal for X-ray detection.

The X-rays used in this experiment were IOOKV, and the thickness of the scintillator crystal was 2 mm.

As a result of this experiment, for crystals with an absorption coefficient of 3crrL-1 or more,
Line detection sensitivity is low, at a few percent.

As shown in Table 2 below, the impurity concentration was reduced to 50
ppm, the absorption coefficient is -1 of 1.8, and when this crystal is used, the X-ray detection sensitivity is 12%, and B 1 4, which has been conventionally used as a scintillator crystal, is
Almost the same X-ray detection sensitivity as G e 30 t was obtained.

Further, when the impurity concentration is 20 ppm and the absorption coefficient is 1.2 crrL-1, the X-ray detection sensitivity is 22%, 32% with an absorption coefficient of 0.5 crrL-1, and 40% with an absorption coefficient of 0.21 cm'. %Met.

Furthermore, even when the absorption coefficient was further reduced, the Xi detection sensitivity remained at approximately 40%.

Example 1 Next, the method for manufacturing the ZnwO4 crystal of the present invention will be described.99. 9 Place 9% high purity ■03 and ZnO oxide in equimolar proportions in a platinum crucible (raw material weight 400g).
r), the raw material is heated by high frequency heating in an oxygen atmosphere to approx.
Heating to 100'C, using Czochralski method,
A ZnWO4 crystal with a diameter of 25 mm could be grown under the conditions of a crystal pulling speed of 4 mml time and a rotation speed of 50 rpm.

The crystal of the present invention thus obtained has a slightly brown color, but the absorption coefficient at a wavelength of 520 nm is 1.
8 cfrL', and the impurity content was 50 ppm.

(Invention ■) The impurities at this time were mainly Si and Ca.

Example 2 Next, Zr manufactured in Example 1 with an impurity amount of 50 ppm
Using LWO4 crystal as a raw material, crystal growth was performed again (the growth conditions were the same as in Example 1), and the resulting crystal was transparent, with an absorption coefficient of 0.21Cr/L-1 and an impurity content of 1.
ZnWo4 crystals with a concentration of 0 ppm could be obtained.

(Invention ■) Comparative Example On the other hand, equimolar amounts of 99.9% pure WO3 and ZnO oxide were mixed into a platinum crucible, and crystal growth was performed by the Czochralski method under the same conditions as in Example 1. The resulting Z nWo crystals were strongly colored and had an absorption coefficient of 4. OCr/'L-1 (conventional product).

The impurity content in this case was 120 ppm, and Si, Ca, and Fe were present as impurities.

This Z nWO 4 crystal could not be used as a scintillator for X-ray detection.

Example 3 When the ZnWO4 crystal produced in the comparative example was bound as raw materials and crystal growth was performed again by the Czochralski method (growth conditions were the same as in Example 1), the absorption coefficient was 1.2 cI.
rL' and X-ray detection sensitivity is also 22%.
(This invention ■)

Example 4 When growing a ZnW04 crystal by the Czochralski method using the same raw materials and crystal growth conditions as in Example 1, an electric field was applied so that the crystal side was negative and the melt side was positive, and the ．． Crystal growth was performed by passing a current of 5 m AicrA, and the resulting crystal had a diameter of 25 r/m.
L1rL and its absorption coefficient is 1.3cm-1
, it was possible to make it even lower than in the case of Example 1.

When this ZnWo4 crystal was used as a scintillator crystal, the X-ray detection sensitivity was as good as 19%.

Also, by the way, the impurity content of this crystal is 3 4 pp
m (present invention ■).

Example 5 ■03 with a purity of 99.99% and ZnO oxide were mixed in equal moles, and 400 gr of this raw material was transferred to a platinum boat (2
0mm x 20mm x 200mm),
Using a ring-shaped resistance heating element made of SiC from around the boat, and moving the heating element in the longitudinal direction of the boat at a moving speed of 4 mml hours, the raw materials in the boat were sequentially melted in a band shape with a width of 257n7 ILO. (melting temperature 12
When the crystal was grown by the zone melt method at 00°C, a ZnW04 crystal with an absorption coefficient of 1.7 crrL-1 was obtained.

When this crystal was used as a scintillator for X-ray detection, the X-ray detection sensitivity was 13% (invention (2)).

A typical example of the case where the ZnWO4 crystal described above is used as a scintillator is compared with the characteristics of conventional scintillator crystals as shown in Table 2 below.

As is clear from the table, the scintillator crystal made of ZnWO of the present invention can have an extremely high X-ray detection sensitivity by setting the absorption coefficient at a wavelength of 520 nm to 1.8 crrL-1 or less. Therefore, it can be said that it is the most suitable scintillator crystal for X-ray tomographic imaging devices in terms of the three points of X-ray detection sensitivity, emission wavelength, and afterglow characteristics, as well as the original emission wavelength afterglow characteristics.

Further, by setting the absorption coefficient to 1.2 crIL' or less, more preferable X-ray detection sensitivity can be obtained, and a more useful crystal for scintillator can be obtained.

As methods for reducing the absorption coefficient, methods such as repeating crystal growth or applying an electric field during crystal growth using the Czochralski method have been described, but it is also possible to use a combination of these methods. Further, in addition to the growth conditions and the like exemplified, it goes without saying that conditions used in normal crystal growth can be used.

Note that although the above description has mainly been about the case where the crystal is used as a scintillator crystal in an X-ray tomographic image pickup device, it is noted that it can also be used for gamma rays.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the absorption coefficient and X-ray detection sensitivity at a wavelength of 520n7A of ZnWO4 crystal.

Claims (1)

[Claims] 1. Z nW having an impurity content of 50 ppml or less
A scintillator crystal characterized by being made of O4. 2. The scintillator crystal according to claim 1, which has an impurity content of 20 ppml or less. 3. The scintillator crystal according to claim 1 or 2, wherein the impurities are mainly Si and Ca. 4. A method for producing a scintillator crystal, which comprises growing a crystal by melting a ZnW04 crystal having an impurity concentration of 50 ppm or less. 5. The method for producing a crystal for a scintillator according to claim 4, wherein the crystal growth is performed by the Czochralski method. 6. The method for producing a scintillator crystal according to claim 4, wherein the crystal growth is performed by a zone melt method. 7. When a ZnW04 raw material with an impurity concentration of 50 ppm or less is melted and a crystal is grown by the Czochralski method. A method for producing a scintillator crystal, characterized by applying an electric field between the side of the crystal to be grown and the side of the melt. 8. The method for manufacturing a scintillator crystal according to claim 7, wherein when applying the electric field, the electric field is applied so that the electric field is negative on the crystal side and positive on the melt side.